1. Field of the Invention
The present invention relates to a rubber product and a method for manufacturing the rubber product.
2. Description of the Art
Conventionally, automotive radiator hoses and other rubber products have been made from a rubber composition containing polymer, such as ethylene-propylene-diene monomer (EPDM), a reinforcing agent, such as carbon black, a vulcanizing agent, such as sulfur, and a plasticizer, such as oil. The rubber composition is typically prepared through a mastication process and a kneading process. In the mastication process, the polymer is plasticized by the application of shearing force to disentangle molecules or the cleavage of molecular linkages. The plasticized polymer is mixed with carbon black, sulfur, oil, and other materials in the kneading process. For further details, see Japanese Unexamined Patent Application Publication No. 10-180941 and “Gomu no Jiten” (an encyclopedia of rubber), edited by Michio Okuyama et al., Asakura-shoten, Tokyo (2000), p. 303.
However, low-polarity rubber (rubber having few polar groups), such as EPDM, has a low affinity for carbon black. Thus, it is difficult to disperse the carbon black uniformly in the rubber. Furthermore, when polar carbon black is used to increase the electrical conductivity of a rubber product, the affinity between the low-polarity rubber and carbon black further decreases.
To achieve sufficient mixing, various procedures, including a masterbatch method, have been proposed. However, even using these mixing procedures, carbon black, in particular the polar carbon black, tends to separate from the polymer and may reaggregate over time. The reaggregation results in surface roughness of a final rubber product, and also leads to reduction or variation in the electrical resistance of the rubber product.
On the other hand, the addition of a silane coupling agent, sulfur or a sulfur-based vulcanization accelerator, and/or a phenol-based stabilizer to the rubber composition is proposed to prevent the separation and the reaggregation of the carbon black. However, the silane coupling agent is expensive and increases the production cost. In addition, unpleasant odor of the silane coupling agent deteriorates the work environment. The silane coupling agent may also deteriorate the moldability of extrusion molding and increase the surface roughness of an extrudate. On the other hand, sulfur or the sulfur-based vulcanization accelerator must be mixed at 150° C. or lower. At temperatures higher than 150° C., where general-purpose compounds are often subjected to short-time mixing, sulfur or the sulfur-based vulcanization accelerator seems to have little dispersion effect on the carbon black. While the phenol-based stabilizer, such as those manufactured by Sumitomo Chemical Co. Ltd. under the trade name of Sumilizer GM or Sumilizer GS, is suitably used owing to its excellent dispersion effect on the carbon black, it is very expensive and is not suitable for general use.